Many applications exist for devices which modulate a light beam, e.g. by altering the amplitude, frequency or phase of the light. An example of such a device is a reflective deformable grating light modulator 10 is shown in FIG. 1. The modulator 10 includes a plurality of equally spaced apart, deformable reflective ribbons 18 which are suspended above a reflective substrate 16. This modulator was proposed by Bloom et al., in U.S. Pat. No. 5,311,360 which is incorporated herein by reference. The ribbons 18 are held on a nitride frame 20 on an oxide spacer layer 12. For modulating light having a single wavelength .lambda..sub.0, the modulator is designed such that the thickness of the ribbons and the thickness of the oxide spacer, both equal .lambda..sub.0 /4.
The grating amplitude of this modulator, defined as the perpendicular distance, d, between the reflective surface on the ribbons 18 layers and the reflective surfaces of the substrate 16, is controlled electronically. In its undeformed state, with no voltage applied between the ribbons 18 and the substrate 16, the grating amplitude equals .lambda..sub.0 /2 and the total path length difference between light reflected from the ribbons and the substrate equals .lambda..sub.0, resulting in these reflections adding in phase and the modulator reflects light as a flat mirror. When an appropriate voltage is applied across the ribbons 18 and the substrate 16, an electrostatic force pulls the ribbons 18 down onto the surface of the substrate 16 and the grating amplitude is changed to equal .lambda..sub.0 /4. The total path length difference is one-half wavelength, resulting in the reflections from the surface of the deformed ribbons 18 and the reflections from the substrate 16 interfering destructively. As a result of this interference the modulator diffracts the light.
Grating light modulators of the type described in the Bloom et al., '360 patent can be used to create a structure for displaying images. A pixel can be formed of such a modulator with a minimum of one pair of adjacent grating elements. Where the display has an optical system which detects only the diffracted light, the pixel is dark or off when no voltage is applied to the ribbon and the ribbon remains in the up position, and the pixel is lighted or on when voltage is applied to the ribbon and the ribbon is pulled down onto the substrate. One very important criteria for designing display systems is the contrast ratio between a dark pixel and a lighted pixel. The best way to provide relatively large contrast ratio is to ensure that a dark pixel has no light.
A method for forming the modulator 10 is proposed in the Bloom et al., '360 patent. Referring to FIG. 1, an insulating layer 11 is deposited on a silicon substrate 16. This is followed by the deposition of a sacrificial silicon dioxide film 12 and a low-stress silicon nitride film 14, both 213 nm thick. Because these thicknesses determine the grating amplitude of the modulator, their dimensions are critical. Variations in these thicknesses will result in unwanted diffraction of light in the off state, thus lower contrast ratios.
The silicon nitride film 14 is then photolithographically patterned and etched into a grid of grating elements in the form of elongated elements 18. After this lithographic patterning process a peripheral silicon nitride frame 20 remains around the entire perimeter of the upper surface of the silicon substrate 16. Then the sacrificial silicon dioxide film 12 is etched in hydrofluoric acid. The silicon dioxide film 12 is completely removed by the acid etch thereby resulting in a plurality of silicon nitride ribbons, 213 nm thick, stretched within the frame and suspended a distance of 213 nm (this being the thickness of the etched away sacrificial film) above the silicon substrate 16.
As can be further seen the silicon dioxide layer 12 is not entirely etched below the frame 20. In this way, the frame 20 is supported, a distance of 213 nm, above the silicon substrate 16 by this remaining portion of the silicon dioxide 12 film. This requires a carefully controlled time-dependent etch to ensure that silicon dioxide layer 12 is left under the frame 20. Next, a 50 nm thick aluminum film is sputtered onto the ribbons 18 and the substrate 16. This aluminum film enhances the reflectivity of both the ribbons 18 and the substrate 16 and provides a first electrode for applying a voltage between the ribbons 18 and the substrate 16. A second electrode is formed by sputtering an aluminum film of similar thickness onto the base of the silicon substrate 16.
Adhesion between the ribbons 18 and the substrate 16 during the final wet processing step and during operation has been found to be a problem in these devices. The force causing this adhesion is a function of the contact area between the two surfaces and the adhesion specific-force (that is force per unit of contact area). Numerous techniques to reduce adhesion have been proposed, including: freeze-drying, dry etching of a photoresist-acetone sacrificial layer, OTS monolayer treatments, use of stiffer ribbons by using shorter ribbons and/or tenser nitride fills, roughening or corrugating one or both of the surfaces, forming inverted rails on the underneath of the ribbons, and changing the chemical nature of the surfaces. Sandejas et al. in "Surface Microfabrication of Deformable Grating Light Valves for High Resolution Displays" and Apte et al. in "Grating Light Valves for High Resolution Displays", Solid State Sensors and Actuators Workshop, Hilton Head Island, S.C. (June 1994), have demonstrated that such adhesion may be prevented by reducing the area of contact by forming inverted rails on the underneath of the bridges and by using rough polysilicon films, respectively. Currently, the preferred technique is to roughen one or both surfaces. However, because the substrate of the modulator 10 is used as an optical surface, the manufacturing processes for roughening the surfaces are complicated by the requirements that the reflecting portions of the substrate 16 be smooth with high reflectivity and be in a plane parallel to the ribbons 18, whereas the portions of the substrate under the ribbons 18 are rough.
The Bloom et al., '360 patent proposes, but does not disclose a method of making, other embodiments of modulators which do not use the substrate as a reflecting surface. One type of modulator 30 is illustrated in FIG. 2 and includes fixed ribbons 38 alternately includes moveable ribbons 34. The fixed ribbons 38 are arranged to be coplanar with the moveable ribbons 34 and thereby present a substantially flat upper surface and the modulator reflects incident light as a flat mirror when no biasing voltage is applied. When a biasing voltage is applied the moveable ribbons 34 move downwards and the modulator diffracts the light. However, this device appears difficult to make, and its performance, like modulator 10 is very sensitive to the thicknesses of the ribbons and the oxide spacers under the fixed elements.
Furthermore, the contrast ratio and intensity of displays consisting of modulators 10 (FIG. 1) and 30 (FIG. 2) are also sensitive to inadvertent periodicity in the grating structure caused by processing. For example, swelling of the oxide spacers supporting the fixed ribbons, may occur during the processing of modulator 30. Such swelling would result in the fixed ribbons and the moveable ribbons not being coplanar in the off state, resulting in light being diffracted instead of reflected.
Furthermore, these modulators are subject to contrast ratio degradation due to effects of applied voltages and noise. In the first case, the presence of applied voltage to the substrate and to specific ribbons which are to be moved down will be felt by the other ribbons which are to remain up (pixel off) and these ribbons will bend and diffract some of the incident light so the pixel is partially on and lit instead of off and dark, resulting in a reduced contract ratio. Likewise the ribbons and not the fixed elements will bend in response to noise. This results in degradation of the contrast ratio due to a) variations in the grating amplitude (distance between the reflective surfaces of the adjacent grating elements) in modulator 10 and b) the ribbons and fixed elements no longer being coplanar in modular 30.
In summary, the performance of the modulators made using the prior art method suffer from, but not limited to, the following drawbacks: performance is very sensitive to process variations because the dimensions for the thickness of the ribbons, the sacrificial layer, and remaining oxide spacers define the grating amplitude; contrast ratio is optimized only for a single wavelength; the dark state and hence the contrast ratio is highly dependent on wavelength; the height of the ribbons over the substrate in the relaxed state cannot be adjusted after processing to tune for different wavelengths or to adjust for manufacturing variations; the preferred method for preventing sticking, roughening both surfaces, degrades the reflectivity of the substrate grating elements; and contrast ratio is degraded due to the effects of applied voltages and noise.
What is needed is a flat diffraction grating light valve which exhibits the following characteristics: the dark-state is independent of wavelength, the contrast ratio for white light operation is relatively high, the grating amplitude can be adjusted to optimize performance, self-biasing, common-mode rejection of noise, simple and cost-effective to manufacture, and tolerant of process variations.
Additionally, a method is needed of manufacturing such light valves and flat diffraction grating systems which exhibits the following characteristics: simple manufacturing process, high yields, self-limiting sacrificial layer etch, self-supporting modulator elements (no frame), and simplifies the process for obviating adhesion. Additionally, substrate manufacturing issues (roughness for reduction of adhesion and conductivity) are decoupled from optical issues (reflectivity and flatness).